1. Field of the Invention
The present invention relates to a hands-free speech communication apparatus and, more particularly, to a hands-free speech communication apparatus incorporating an echo canceler in, e.g., a mobile telephone.
2. Description of the Prior Art
Referring to FIG. 1, a hands-free speech communication apparatus as the first prior art which incorporates an echo canceler and is used for a mobile telephone is realized by simply connecting, as a hardware structure, a device 9 dedicated to the echo canceler, and a voice switch 7 and a line gain correction circuit 6 which are realized by discrete parts and the like. That is, the circuits of the device 9 dedicated to the echo canceler, the voice switch 7, and the line gain correction circuit 6 independently operate.
As the second prior art, a "howling prevention apparatus" disclosed in Japanese Unexamined Patent Publication No. 5-75500, i.e., a prior art of a hands-free speech communication apparatus used for a TV conference system and incorporating an echo canceler will be described. Referring to a block diagram in FIG. 2, the howling prevention apparatus for a bidirectional speech communication apparatus having an open-loop closed circuit constituted by acoustic coupling between a microphone 31 for inputting a transmission voice and a loudspeaker 34 for outputting a reception voice comprises an echo canceler 35 which generates a pseudo echo from an output signal from the bidirectional speech communication apparatus and subtracts this pseudo echo from an input signal to the bidirectional speech communication apparatus, thereby removing an echo passing through the open-loop closed circuit, an attenuation amount calculation unit 36 for calculating, on the basis of the acoustic coupling gain, the echo cancel amount of the echo canceler 35, and the amplified gain of the open-loop closed circuit, an attenuation amount for setting the gain of the open-loop closed circuit to one or less, and a voice switch 37 for inserting/controlling, into/in the open-loop closed circuit, the attenuation amount calculated by the attenuation amount calculation unit 36. The echo canceler 35 calculates a coefficient by a coefficient estimating operation and multiplies the calculated coefficient with the output signal from the bidirectional speech communication apparatus, thereby generating the pseudo echo.
The acoustic coupling gain is approximately calculated on the basis of the converged value of the coefficient of the echo canceler 35. The echo cancel amount is approximately calculated by obtaining the power ratio between the input signal to the echo canceler and the residual echo.
The voice switch 37 forcibly inserts an infinite attenuation amount regardless of the calculation result from the attenuation amount calculation unit 36 until the attenuation amount calculated by the attenuation amount calculation unit 36 decreases to a value for allowing concurrent bidirectional speech communication. When the attenuation amount calculated by the attenuation amount calculation unit 36 is larger than the value for enabling concurrent bidirectional speech communication, the voice switch 37 divides the voice signal transmission band into a plurality of bands and independently sets the attenuation amount to be inserted in units of divided bands. In addition, when the attenuation amount calculated by the attenuation amount calculation unit 36 is larger than the value for allowing the concurrent bidirectional speech communication, the voice switch 37 performs Fourier transform of the voice signal and independently sets the attenuation amount to be inserted in units of frequency components obtained upon Fourier transform.
Each of these conventional hands-free speech communication apparatuses incorporating echo cancelers requires the device dedicated to the echo canceler and the voice switch circuit realized by discrete parts, resulting in a large circuit scale and high cost. The echo canceler and the voice switch independently operate. For this reason, when the loss of the voice switch is reduced in consideration of the cancel amount of the echo canceler, howling may be generated before the echo canceler exhibits a sufficient effect. When the loss of the voice switch is set to control the howling, the loss of the line becomes too large while the echo canceler exhibits a sufficient effect, and the concurrent speech communication properties undesirably degrade.
In each of these conventional hands-free speech communication apparatuses incorporating echo cancelers, the gain, time constant, operating point (threshold), and the like of each circuit must be changed for each country because of the difference between line gains in various countries.